1. Field of the Invention
The present invention relates to a method of manufacturing a light emitting device which has a light emitting element in which an emitter composed of an organic compound is provided between a pair of electrodes, and to a printing device which is applicable to the manufacture of the light emitting device.
2. Description of the Related Art
A light emitting element in which an organic compound is used for an emitter (hereinafter referred to as organic light emitting element) generally has a sandwich structure in which an organic compound layer is formed between a cathode and an anode. In case of using a low molecular weight organic compound, the structure is formed by combining layers such as a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer, which are composed of respective materials. The hole injecting layer and the hole transporting layer are not necessarily strictly distinguished, and the hole transporting property (hole mobility) is a particularly important characteristic for both the layers. For the sake of convenience, the hole injecting layer corresponds to a layer that contacts an anode, and a layer that contacts the hole injection layer is called the hole transporting layer. Thus, both the layers are distinguished from each other. Also, a layer that contacts a cathode is called the electron injecting layer, and a layer that contacts the electron injecting layer is called the electron transporting layer. The light emitting layer may also serve as the electron transporting layer, and thus, is also called a light emitting electron transporting layer. A vacuum evaporation method is adopted for forming a layer composed of the low molecular weight organic compound.
On the contrary, a polymer organic compound material is formed by a wet method such as an application method or a printing method. Recently, an ink jet printing method has attracted attention, and the technique is disclosed in JP 10-12377 A, JP 10-153967 A, JP 11-54270 A, and the like. As the organic compound material used in the ink jet method, a water-soluble, or alcohol or glycol-soluble material is applied, and a precursor of polyparaphenylene vinylene (hereinafter referred to as PPV) or the like is used therefor. The PPV precursor becomes salt and dissolves in water, and is polymerized to become a high polymer by heating after application.
A light emitting mechanism in the above-mentioned organic compounds is regarded as a phenomenon in which: an electron injected from a cathode and a hole injected from an anode recombine in a light emitting layer composed of an emitter to form a molecular exciton; and light is emitter when the molecular exciton returns to a base state. There are fight emission from a singlet excitation state (fluorescence) and light emission from a triplet excitation state (phosphorescence). A luminance ranges from several thousands to several tens of thousands cd/m2. Thus, it is considered that the light emitting mechanism can be sufficiently applied to a display device and the like in principle.
It is impossible in terms of heat-resistance or water-resistance that an organic compound layer is formed by patterning with an exposure process like semiconductor. The organic compound layer is formed by patterning with a shadow mask in a vacuum evaporation method. In this case, an organic compound material attached to the shadow mask is not utilized and lost. Besides, the organic compound material that remains on an inner wall of a chamber of an evaporation device and in a crucible of an evaporation source becomes a loss. Thus, the utilization efficiency is expected to be approximately 10%. Therefore, the low molecular weight organic compound layer formed by the vacuum evaporation method has involved a problem in that the utilization efficiency of the material is poor.
Of course, it may be sufficient that the material cost is low. However, the low molecular weight organic compound material needed to have higher purity is expensive, which is reflected in the cost of a product.
On the other hand, when an organic light emitting element is formed by the ink jet method using a polymer organic compound, there is an advantage that the patterning formation step is simple and easy. An organic compound layer is formed for each dot, whereby the steps such as photoresist patterning and etching are not required. Further, the utilization efficiency of the material by far increases, and it becomes possible to secure the utilization efficiency of 80% or more.
However, as a multiple production method in which a plurality of display panels are taken out from one large-area substrate, it can not be said that the ink jet method in which a mixture is dropped each dot has always high capacity as an effective production method. The ink jet method has involved a problem in that a processing time increases after all since higher accuracy of position and higher processing speed are demanded as a screen size becomes larger and the number of pixels increases.
Further, in the case where the organic compound layer is formed by dropping the mixture every dot, variation in a discharge amount of the mixture and fluctuation of a drop position lead to unevenness in emission intensity, which causes degradation of quality of a light emitting device constituted of a large number of light emitting elements.